Sylvester. O. Adejo1&2*, M. M. Ekwenchi2, John U. Ahile1, Joseph. A. Gbertyo1, Benjamin Ishua1, Andrew Akombor1
Keywords: Adsorption, Mild steel, Manihot esculentum, Adejo-Ekwenchi isotherm, physisorption
Abstract:
The best fit adsorption isotherm model for methanol leaf extract of Manihot esculentum as green corrosion inhibitor of corrosion of mild steel in 2 M HCl medium was determined with the help of the Adejo-Ekwenchi adsorption isotherm. The corrosion inhibition study was carried out using the weight loss method at the temperature range of 301 K to 313 K. The inhibition efficiency, %IE, of the extract was found to increase with increase in both concentration and temperature, suggestive chemisorption. Going by the values of %IE, R2 and ΔGads this adsorption process would have been thought to be chemisorption and well fitted into Langmuir, Freundlich and Temkin isotherm models. However, from the information obtained through the Adejo-Ekwenchi isotherm model the adsorption process was unambiguously resolved to be physisorption and best fitted into the Langmuir isotherm model.
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MODELING OF ADSORPTION ISOTHERM FOR METHANOL LEAF EXTRACT OF Manihot esculentum AS GREEN CORROSION INHIBITOR OF CORROSION OF MILD STEEL IN HCl MEDIUM Sylvester. O. Adejo1&2*, M. M. Ekwenchi2, John U. Ahile1, Joseph. A. Gbertyo1, Benjamin Ishua1, Andrew Akombor1 Physical Chemistry Unit, Department of Chemistry, Benue State University, Makurdi-Nigeria 2 Department of Pure and Applied Chemistry, University of Jos, Jos-Nigeria Author of Correspondence: sadejo@bsum.edu.ng +2348054557652 Abstract The best fit adsorption isotherm model for methanol leaf extract of Manihot esculentum as green corrosion inhibitor of corrosion of mild steel in 2 M HCl medium was determined with the help of the Adejo-Ekwenchi adsorption isotherm. The corrosion inhibition study was carried out using the weight loss method at the temperature range of 301 K to 313 K. The inhibition efficiency, %IE, of the extract was found to increase with increase in both concentration and temperature, suggestive chemisorption. Going by the values of %IE, R 2 and ΔGads this adsorption process would have been thought to be chemisorption and well fitted into Langmuir, Freundlich and Temkin isotherm models. However, from the information obtained through the Adejo-Ekwenchi isotherm model the adsorption process was unambiguously resolved to be physisorption and best fitted into the Langmuir isotherm model. 1 Keywords: Adsorption, Mild steel, Manihot esculentum, Adejo-Ekwenchi isotherm, physisorption Introduction Adsorption has been adjudged to be an effective process for a wide variety of applications [1, 2]. Empirically, the process of adsorption can be described in terms of amount adsorbed as a function of pressure (or concentration) and temperature [3, 4]. For the adsorption process, the relationship between the amount of the adsorbed substance to its bulk concentration at a particular temperature and pH is known as adsorption isotherm, and many adsorption isotherms have been proposed with their characteristic parameters. The importance of the parameters of adsorption isotherms lay in the fact that they useful for the deduction of the mechanism of adsorption as well as for design purposes [1, 5-9]. Several error analysis methods such as coefficient of determination, the sum of the errors, a hybrid error function, Marquardt‟s per cent standard, the average error and sum of absolute error, have been employed in the determination of the best-fitting isotherm for any adsorption process [1]. Among these, the method of least squares with linearly transformed isotherm equations has be acclaimed as the most widely used for finding the best fit of adsorption isotherm [10]. Also the Gibb‟s free energy, ΔGads, value is usually employed to ascertain whether the adsorption process is physisorption or chemisorption. But, Ho and Co-workers [5] have noted the inadequacy of usage of coefficient of determination, R 2, for the determination of best isotherm fit thus “the transformation of non-linear isotherm equations to linear forms implicitly alters their error structure and may also violate the error variance and normality assumptions of standard least squares”. Again, it has been stated by many authors that value of ΔGads below -20 kJ/mol is an indication of physisorption, while above -40 kJ/mol is ascribed to be chemisorption. However, Popova and his research team [10] argued that the use of ΔGads as a sole criterion for such distinction will be difficult, especially in the case where charged species are adsorbed due to possibility of coulombic interaction between charged species; it can increase the ΔGads value, even if no new chemical bonds are formed. Evidently, therefore, there exist ambiguities in the usage of R2 and ΔGads values as sole criteria in the determination of isotherm best fit of adsorption process. Unambiguously, it has been shown that the AdejoEkwenchi isotherm can be used to eliminate the observed inadequacies in adsorption characterisation process, especially in corrosion inhibition using plant extracts [12- 14, 13]. In this present paper we unambiguously resolved the adsorption isotherm best fit for methanol leaf extract of Manihot esculentum as green corrosion inhibitor of corrosion of mild steel in 2 M HCl medium. Research Methods Material Preparations Leaves of Manihot esculentum were collected and rinsed with distilled water, then shade-dried and pounded into powder. 30 g of the powder was soaked in 300 mL of 98% methanol (BDH Chemicals Ltd, England) in a 500 mL volumetric flask for 48 hours, with occasional shaking. The extract obtained therefrom was filtered using glass wool and the methanol was slowly evaporated using thermostated water bath (Clifton: Nickel-Electro Ltd, England) at 321 K for reason given elsewhere [15]. The residue obtained was preserved in a desiccator. Stock solutions of 0.1, 0.2, 0.3, 0.4 and 0.5 g/dm3 concentrations were prepared in 2 M HCl (M & B). Coupons (2 cm x 2 cm x 0.13 cm each) were prepared as reported before [16] from a sample of mild steel of composition (%W): Fe(98.84), Mn(0.56), P(0.04), C(0.27), Si(0.25) and S(0.04). The coupon surfaces were thoroughly polished to mirror finishing using different grades of emery paper, degreased by washing with ethanol (BDH), dried in acetone, and preserved in a desiccator for use. Experimental Methods The coupons, degreased in acetone (99% BDH Chemicals Ltd, England), were carefully weighed using ae Adam AFP electronic weighing balance (d ±0.0001g) and immersed in 50 mL of the acid solutions, without and with various concentrations of the inhibitor at the temperature range of 301 K to 313 K in the water bath for 8 hours. At the retrieval of each coupon, the reaction was terminated by dipping into saturated solution of ammonium acetate (97% Labtech Chemicals Limited) as outlined by Orubite-Okorosaye and Oforka [17] and the coupon stored in the desiccator until ambient temperature was attained and reweighed. An average of triplicate values was taken for each measurement. The %IE and surface coverage ( ) were calculated through equations (1) and (2), respectively [18-21]. (1) (2) where respectively. and are weight losses in absence and present of inhibitor, Results and Discussion Table 1 shows the variation of %IE of the methanol leaf extract of Manihot esculentum as eco-friendly corrosion inhibitor of the corrosion of mild steel sample in 2 M HCl with the extract concentration and temperature. The Table clearly shows that the %IE increases with increase in both concentration of the extract and rise in temperature. The increase in the %IE with rise in temperature is suggestive of chemisorption [22, 23]. Table 1. Variation of Inhibition Efficiency (%IE) with Concentration and Temperature of Leaf Extract of Manihot esculentum Conc. (g/dm3) %IE 301 K 305 K 309 K 313 K 0.1 0.2 0.3 0.4 0.5 29.28 44.77 58.08 77.21 79.19 47.40 60.86 70.32 77.87 82.54 51.35 66.66 73.63 77.31 84.40 52.76 65.22 74.49 78.77 85.06 The inhibitive action of organic compounds has been ascribed to the formation of surface layers and films on the metal surface, thereby reducing the accessibility of the corrodant to the metal surface. Adsorption isotherms have been used to characterise this inhibitive action. The most frequently used of such isotherms are the Langmuir, Freundlich, Frumkin, Temkin, Flory-Huggins, Sips, Bockris-Swinkels and the El-Awady thermodynamic-kinetic model and the recent proposed Adejo-Ekwenchi isotherm [1, 12, 24, 25]. The linearised forms of the various equations are given below. Langmuir (3) Freundlich (4) Frumkin [ Flory-Huggins log (5) ] = log K + xlog [1 - θ] (6) Temkin El-Awady (7) log = log K’ + ylog [C] Sips Adejo – Ekwenchi (8) [ ] [ ] (9) (10) The fitness adsorption data to an adsorption isotherm has been determined by many authors through the use of the value of regression coefficient, R2, from the plot of the surface coverage against the bulk concentration of the adsorbate (extract). Going by this it can be asserted that the data for this extract fitted into the Langmuir, Freundlich, Temkin, El-Awady and Adejo-Ekwenchi isotherms (Table 2), not fitting other popular two-parameter isotherm like Flory-Huggins, Frumkin and El-Awady. The equilibrium constant, Kads, values of the isotherms (Table 2) are all positive, implicitly, indicative of favourable adsorption in all the isotherms [8]. The Sshaped curve of the plot of %IE against the logarithm of concentration in Figure 1 (at 301 K) further attests to the fact that an adsorbed layer was actually formed on the metal surface by the extract [8, 26]. Table 2. Various Adsorption Isotherm Parameters for Adsorption of Leaf Extract of Manihot esculentum onto the mild steel surface Isotherm Temp (K) R2 Slope Intercept Kads 301 305 309 313 0.9948 0.9949 0.9393 0.9815 0.6512 0.9748 1.0128 0.9977 0.2955 0.1247 0.0985 0.0996 3.3841 8.0192 10.1523 10.0402 301 305 309 313 0.9876 0.9986 0.9839 0.9967 0.6492 0.3492 0.2979 0.2945 0.1128 0.0270 0.0172 0.0190 1.2966 1.0641 1.0404 1.0447 301 305 309 313 0.9589 0.9967 0.9909 0.9956 0.4820 0.3465 0.3343 0.1754 -3.0297 -4.4070 -4.9053 -4.9284 0.0483 0.0331 0.0074 0.0072 n 0.6492 0.3492 0.2979 0.2945 α -1.4597 -2.2582 -2.5189 -2.5143 Adejo- Ekwenchi 301 305 309 313 0.8908 0.9582 0.9540 0.9507 0.7995 0.6765 0.6532 0.6805 0.8890 0.9205 0.9455 0.9728 7.7446 8.3272 8.3206 9.3929 b 0.7995 0.6765 0.6532 0.6805 ΔGads (kJ/mol) Langmuir Freundlich Temkin -1.5 -1 -0.5 logConc. -10.70 -10.34 -10.42 -10.57 -2.47 -5.84 +2.29 +2.39 -15.18 -15.56 -15.91 -16.28 %IE 100 80 60 40 20 0 -13.10 -15.47 -16.28 -16.46 0 Figure 1. Relationship between Inhibition Efficiency and logarithm of Concentration at 301K for the Extract of Manihot esculentum Equations (11) and (12) are conventionally used to calculate ΔGads for an adsorption process. (11) where (12) C is the concentration of the extract and 55.5 is concentration of water expressed in moles (i.e. ~ 1000 g/dm3) [12, 27]. The values of ΔGads evaluated through the equation (11) are presented in Table 3. The slope and intercept of ideal Langmuir isotherm plot should be unity and zero, respectively [28, 29]. The slopes of the Langmuir plot obtained are close to unity and intercepts almost equal to zero, with good R2 values, indicative of suitability of the Langmuir isotherm model to the adsorption behaviour of this extract. Moreso, the values of ΔGads obtained through this isotherm are also in good agreement with those evaluated through the conventional method as shown in Table 3. Langmuir isotherm is applicable to both physisorption and chemisorption. .Table 3. Values of ΔGads obtained through equilibrium constant Conc.(g/dm3) - ΔGads (kJ/mol) 301 K 305 K 309 K 313 K 0.1 0.2 0.3 0.4 0.5 13.77 13.56 13.88 15.40 15.13 15.76 15.39 15.43 15.70 15.88 16.38 16.24 16.05 15.82 16.44 16.73 16.28 16.38 16.25 16.78 The parameter n of the Freundlich isotherm relates to the adsorption intensity, and it varies with the heterogeneity of the material [30] and it has a typical value is 0.6 [24], of which no value is close (Table 2), except at 301 K. And the values of ΔGads obtained through this isotherm model are also not comparable to those of the conventional method. Therefore, the adsorption process of leaf extract of Manihot esculentum onto the metal sample in HCl cannot be modelled by the Freundlich isotherm, in spite of the seemly good values of R2. The Temkin isotherm mostly is a feature of chemisorption [31] and an indication of interaction of uncharged molecules on a heterogeneous surface [32]. The negative values of parameter α are clear indication of the existence of repulsive interaction among the molecules of the adsorbed layer. The values of R 2 are good, but those of ΔGads show no favourable adsorption by this extract onto the metal surface, a complete contrast to the observation above, and hence cannot be modelled by Temkin isotherm. The Adejo-Ekwenchi isotherm is centred on the fact that for an adsorption process, the amount of adsorbate uptake from bulk concentration has inverse relationship with the difference between the total available surface on the adsorbent surface and the fraction that is covered by the adsorbate at a given temperature, prior to the attainment of maximum value of surface coverage [12]. 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